Heinrich Events in the Southern Hemisphere: A Tropical Trigger?

Heinrich events, or periodic pulses of ice-rafted debris into the North Atlantic, have long been envisioned as having a high-latitude trigger. This trigger is most often cited as internal ice-sheet dynamics. McIntyre and Molfino (1996) proposed a tropical trigger for Heinrich Events: they suggested that precession-related strengthening of the North African monsoonal atmospheric circulation would alter oceanic circulation in such a way as to reduce heat transport to the high latitudes. Discovery of possible evidence (Mg/Ca ratios and faunal abundance) for Heinrich Event 1 (H1) in the Benguela upwelling region (Farmer et al. 2005) stimulated a search for a Heinrich Event 2 (H2) signal. From the review by Hemming (2004), we estimated the time horizon in which we would expect to find H2 as 22.4 - 26.8 ka. We extrapolated the published ODP1084B age model (Farmer et al. 2005) to estimate the depth interval of this time horizon, and counted the relative abundance of the four dominant species of planktonic foraminifera (G. inflata, G. bulloides, N. pachyderma right- and left-coiling) in each sample within this interval. We found an interval spanning five centimeters, or approximately 22.9 to 23.1 ka, in which the relative abundance of N. pachyderma (left-coiling) rose to a an average of 31.5%. Compared to the average level of 23.5% in the surrounding samples, and our pooled standard deviation of 3.4%, this shift appears significant. Increases in relative abundance of N. pachyderma (left-coiling) in earlier intervals are associated with decreased Mg/Ca-based sea surface temperatures (SSTs) and are inferred to be a likely result of increased upwelling (Farmer et al. 2005). Although this new interval is shorter than would be expected from Heinrich event studies elsewhere, the shift in relative abundance of N. pachyderma (left-coiling) is consistent with an H2 signal. Better age control is needed to constrain the timing of this event, however, and data from an additional proxy such as Mg/Ca is needed to confirm the signal.